1. Field of the Invention
The present invention relates to a circuit technology for preventing data held in a register or the like from disappearing at the time of power shutdown, reducing significantly a time for turning power on again, and decreasing leak current when the power is turned on.
2. Description of the Related Art
In recent years, process miniaturization and a reduction in power supply voltage have been required as high-speed and low power consumption of semiconductor devices are achieved. On the other hand, with a reduction in power supply voltage, a reduction in a threshold value of CMOS also is required in order to achieve high-speed operation of circuits. In this case, power consumption due to off-leak current increases, so that another problem is caused in that stand-by time in mobile equipment such as portable phones is decreased. In order to decrease the off-leak current, a power shutdown technique is required.
The power shutdown technique to reduce off-leak current and achieve low power consumption gradually has been put into practice. FIG. 10 is a schematic diagram showing partially a circuit block of a conventional semiconductor device that performs power shutdown. In FIG. 10, reference numeral 101 denotes a clock generating circuit for generating a clock CLK, reference numeral 102 denotes a data holding circuit such as a register for holding data at a timing of a clock CLK from the clock generating circuit 101, reference numeral 103 denotes a logic circuit, reference numeral 104 denotes a shutdown power line through which a power supply voltage VDD1 is supplied/shut down, and reference numeral 107 denotes a ground line that is pulled to a ground potential VSS. As shown in FIG. 10, the conventional power shutdown is performed simply by stopping application of the power supply voltage VDD1 from the outside to the power line 104.
However, the conventional power shutdown method has the following problems.
First, the data held in the data holding circuit 102 such as a resistor may disappear when the power is shut down. That is to say, in a circuit having a function of holding data by voltage supply from the power line 104, the data disappear upon power shutdown. Therefore, when power is turned on again, the state at the time of the power shutdown is not retained at all, and the system is in an initialized state, which is the same state when the system is started up. For this reason, there is a circuit having a function of shifting data in a circuit in which power shutdown does not occur and holding the data temporarily at the time of power shutdown, and transferring the data back after power is turned on again. However, this method requires time for saving data so that it cannot be used in a real-time system, which requires real-time properties.
Secondly, each node of a circuit in which power is shut down is undefined, and when power is turned on again in this state, the undefined node causes a through-current in transistors. Therefore, the time for power rising increases and thus the through-current also increases. This vicious cycle is repeated when power is turned on. In this case, power consumption due to the through-current is increased.